1. Field of the Invention
The present invention relates to a condenser microphone that provides a balanced output of an audio signal by a condenser microphone unit configured by a diaphragm and a fixed electrode.
2. Description of the Related Art
In a condenser microphone, an audio signal is generated on the basis of a change in capacitance between a diaphragm and a fixed electrode opposite to each other.
That is, a condenser microphone unit obtained by arranging the diaphragm opposite the fixed electrode is configured to extract the audio signal using an impedance converter by a field effect transistor (FET), for example, because output impedance is very high, when the capacitance is about several tens of picofarad (pF).
When the condenser microphone unit is connected to the impedance converter, a circuit structure in which one of the fixed electrode and the diaphragm is connected to a reference potential point (ground line) of a circuit and the other is connected to an input terminal of the impedance converter, that is, a gate terminal of the FET is adopted.
In addition, the impedance converter is generally configured by a source follower circuit that obtains an audio output from a source terminal of the FET.
In addition, this kind of condenser microphone including the impedance converter is configured such that the audio signal is supplied to an external apparatus such as a mixer circuit or a microphone amplifier through a balanced shield cable and an operation current is supplied from a known phantom feeding device included at the side of the external apparatus to the side of the condenser microphone through the balanced shield cable.
That is, the condenser microphone using the phantom feeding device adopts measures to transmit a balanced output obtained by configuring the audio signals in a reverse phase relation through the balanced shield cable to minimize an influence of external noise.
FIG. 6 illustrates an example according to the related art using a phase inversion circuit as a unit for providing a balanced output of audio signals from a condenser microphone.
A reference numeral U1 illustrated in FIG. 6 shows the condenser microphone unit. In this example, a condenser microphone unit U1 of an electret type is configured to include an electret dielectric film in any one of a diaphragm and a fixed electrode. In addition, for example, the diaphragm configuring the condenser microphone unit U1 is connected to a reference potential point of a circuit and the fixed electrode is connected to an impedance converter 11.
Current of an audio signal generated by the impedance converter 11 is amplified in a buffer circuit 12 including an emitter follower circuit and is supplied to an inverter circuit 13.
In addition, balanced outputs of a non-inversion audio output from the buffer circuit 12 and an inversion audio output through the inverter circuit 13 are provided to a hot-side terminal Out (+) and a cold-side terminal Out (−) through a buffer circuit 14 receiving an operation current from a phantom feeding device, respectively, and are transmitted to an external apparatus such as a mixer circuit, for example, through a balanced shield cable (not shown) in which a ground line GND is connected to a shield.
In addition, a direct-current operation current supplied from the phantom feeding device (not shown) of the side of the external apparatus is received in the buffer circuit 14 through the balanced shield cables of the hot side and the cold side and the direct-current operation current is supplied to a constant voltage power supply circuit 15. In addition, an output voltage from the constant voltage power supply circuit 15 is used as an operation power supply of the individual circuits shown by the reference numerals 11 to 14.
The condenser microphone that provides a balanced output of the audio signals using the inverter circuit (phase inversion circuit) 13 illustrated in FIG. 6 is disclosed in JP 62-103390 U.
FIG. 7 illustrates an example according to the related art using a transformer as a unit for providing a balanced output of audio signals from a condenser microphone.
Reference numerals U1, 11, and 12 illustrated in FIG. 7 show elements executing the same functions as the functions of the individual units shown by the same reference numerals as the reference numerals illustrated in FIG. 6 and a detailed description thereof is omitted.
In a structure illustrated in FIG. 7, a transformer T1 with a center tap provided in a secondary winding is adopted and an audio output from the buffer circuit 12 is supplied to a primary winding of the transformer T1. In addition, balanced outputs of audio signals that are generated in both ends of the secondary winding of the transformer T1 and are in a reverse phase relation to each other are provided to a hot-side terminal Out (+) and a cold-side terminal Out (−) and are transmitted to an external apparatus such as a mixer circuit through a balanced shield cable (not shown) including a ground line GNU.
In addition, a direct-current operation current supplied from a phantom feeding device (not shown) of the side of the external apparatus is received in the secondary winding of the transformer T1 through the balanced shield cable and is supplied to the constant voltage power supply circuit 15 through the center tap of the secondary winding. In addition, an output voltage from the constant voltage power supply circuit 15 is used as an operation power supply of the individual circuits shown by the reference numerals 11 and 12.
A condenser microphone that provides a balanced output of the audio signals using the transformer T1 illustrated in FIG. 7 is disclosed in JP 2006-352622 A.
According to the condenser microphone disclosed in JP 62-103390 U and illustrated in FIG. 6, all of the condenser microphone unit U1, the impedance converter 11, the buffer circuit 12, the inverter circuit 13, and the buffer circuit 14 configure an unbalanced signal transmission line using the other side as a ground line.
Therefore, in the individual circuits shown by the reference numerals U1 and 11 to 14 illustrated in FIG. 6, when, for example, high frequency noise or the like is overlapped, the noise cannot be removed and particular measures are necessary to suppress external noise.
In addition, according to the structure illustrated in FIG. 6, because the inversion audio output is generated through the inverter circuit 13, unbalance of signals by a difference of signal transmission systems occurs between the inversion audio output and a non-inversion audio output not transmitted through the inverter circuit 13, which results in causing a quality of the audio signals to be deteriorated.
On the other hand, according to the condenser microphone disclosed in JP 2006-352622 A and illustrated in FIG. 7, the individual circuits including the condenser microphone unit U1, the impedance converter 11, the buffer circuit 12, and the primary winding of the transformer T1 configure an unbalanced signal transmission line using the other side as the ground line. For this reason, the condenser microphone is vulnerable to the external noise, similar to the example illustrated in FIG. 6.
In addition, according to the structure illustrated in FIG. 7, entire performance of the microphone is limited by a unique frequency characteristic or a distortion characteristic of the transformer T1, and the transformer T1 increases a cost.